The present invention relates to a color image forming apparatus wherein a toner image is formed on a belt-like image forming means through an electrophotographic system and then the toner image is transferred onto an image-transfer material to be an aimed image.
A number of methods and apparatuses have been proposed for obtaining a color image through an electrophotographic method. As is disclosed in Japanese Patent Publication Open to Public Inspection No. 100770/1986 (hereinafter referred to as Japanese Patent O.P.I. Publication), for example, a method exists wherein latent images corresponding to the number of separated colors of an original image are formed and developed on a photoreceptor drum that is an image retainer, and after each development for each color image, the color image is transferred onto an image-transfer drum to form a multi-color image thereon, said multi-color image being transferred onto a recording paper to be a color copy. An apparatus for this method requires not only a photoreceptor drum but also an image-transfer drum having a size capable of accepting thereon a whole image of an entire page, resulting in a large size and a complicated structure.
As is disclosed in Japanese Patent O.P.I. Publication No. 149972/1986, for example, a further method exists wherein latent images corresponding to the number of separated colors of an image on an original are formed and developed on a photoreceptor drum, and after each development for each color image, the color image is transferred onto an image-transfer material to form a multi-color image thereon. In this method, it is difficult to register or superpose, accurately in position, images each having different colors and thereby it is impossible to obtain a color copy of good quality.
A further method exists wherein formation of a latent image and its development with a color toner are repeated for the number of times corresponding to the number of separated colors of an image on an original, and after registering images each having different colors on a photoreceptor drum, they are transferred and thus a color image is obtained. The basic processes for forming multi-color images in aforesaid method are disclosed in Japanese Patent O.P.I. Publication Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985.
In such a multi-color image forming apparatus wherein a color image is obtained through registering of images, there are provided a plurality of developing units each having therein different color toners around a photoreceptor drum, and the photoreceptor drum is generally rotated repeatedly to develop latent images thereon. Thus, a color image is obtained.
With regard to an image forming means, there has also been proposed, in addition to a photoreceptor drum on the peripheral surface of which a photoconductive substance is coated or evaporated as stated above, a belt-like image forming means wherein a photoconductive substance is coated or provided on a flexible belt. Since the shape of the belt-like image forming means (hereinafter referred also as a photoreceptor belt) is determined when it is spread between rotating rollers including a driving roller, the photoreceptor belt is effective when making, by utilizing the space effectively, a color image forming apparatus that is compact in size. Further, since the photoreceptor belt is capable of running along a path whose radius of curvature is small, it is possible to prevent a failure of separation.
In the color image forming apparatus mentioned above, image forming means such as a charging means, an exposure means, a plurality of developing units and a cleaning means are provided detachably in the apparatus as an independent unitized process cartridge including aforesaid photoreceptor belt so that the image forming means are arranged below the lower side of aforesaid photoreceptor belt to face the lower surface of the belt.
However, when many image forming means are unitized in a cartridge, the cartridge tends to be large in size and heavy in weight, thus it is difficult to mount it on or dismount it from the main body of the apparatus, and it is inconvenient from the viewpoint of maintenance because each image forming means has its own life or maintenance frequency which differs from others. Further, it is generally necessary to provide a tension roller for the purpose of keeping the photoreceptor belt spread with tension.
The first object of the invention is to provide a color image forming apparatus wherein a photoreceptor belt that is spread with tension and a cleaning means are unitized for maintenance, and other image forming means can be mounted on or dismounted from the main body of an apparatus easily without any interference.
The invention also relates to an image forming apparatus wherein it is possible to select any of a color mode in which toner images of different colors are superposed on a rotating photoreceptor surface and a color toner image thus formed is transferred, and a single color mode in which a single color toner image is formed on a rotating photoreceptor surface and then is transferred from the photoreceptor surface. In this type of image forming apparatus, there have been known an apparatus employing a cleaning means whose blade is caused to be in contact, partially at its end portion, with the surface of a photoreceptor. Toner thus scraped off the surface onto the blade is caused to drop from the other end portion of the blade.
In the conventional image forming apparatus mentioned above, from the viewpoint of the life of a photoreceptor and that of a blade, the blade of a cleaning means is kept away from the surface of the photoreceptor in a non-operation position before the start of an image forming process in which the photoreceptor is stationary; and after the start of the image forming process in which the photoreceptor is rotating in any mode selected, one end portion of the blade of the cleaning means in many cases is caused to be in contact with the surface of the photoreceptor. When a single color mode is selected, the cleaning means is kept in its operation position during the period that a single color toner image forming and its transfer are repeated for the specified number of times and the photoreceptor makes one turn or more before it stops, thus the image forming process is completed. When a color mode is selected, the cleaning means is returned to its non-operation position before the leading edge of a component color toner image formed first on the photoreceptor surface through one turn or more of the photoreceptor reaches the cleaning means, and that position is maintained until the trailing edge of a component color toner image preceding the last image finishes passing the position of the cleaning means. Then, the cleaning means is caused to be in the operation position before the leading edge of the photoreceptor surface from which the color toner image has been transferred reaches the cleaning means, and that position is maintained until the photoreceptor makes at least one turn. After that, when the specified number for color toner forming is one, the image forming process is completed similarly to that of the single color mode, while, in the case of a plural specified number for color toner image forming, the photoreceptor continues rotating and the cleaning means is switched repeatedly as described above until the last color toner image forming and its transfer are completed, thus the image forming process is finished finally as in the case where the specified number is one.
In the image forming apparatus mentioned above, on the other hand, due to the various restrictions, the blade of the cleaning means sometimes can not be slanted fully to prevent toners from staying on the blade on condition that one end portion of the blade is in contact with the photoreceptor surface. This tends to happen when a belt-like photoreceptor whose lower side is in tension and upper side is loose is used as a photoreceptor, a plurality of developing units are provided below the belt-like photoreceptor, transfer is performed at the position of a driving roller for the belt-like photoreceptor, and cleaning is performed at the position of a driven roller, for the purpose of a compact image forming apparatus, less toner contamination, standardization of a plurality of developing units, same conditions for photoreceptors and the structure for the easy opening and access to the path of image-transfer paper.
Even when the blade of a cleaning means can not be slanted fully in its operation position in the image forming apparatus as stated above, the cleaning means is switched to its non-operation position for each color toner image formation and thereby the blade leaves the photoreceptor and inclines fully when a color mode is selected. Therefore, toner staying on the blade until that time drop, preventing a large amount of toner from staying on the blade. When a single color mode is selected, however, the cleaning means is not switched to its non-operation position during the period of single color toner image formations for the specified number of times. When the specified number is large, therefore, a large amount of toner stay on the blade, resulting in elastic deformation of the blade, deterioration of cleaning capability and occurrence of toner contamination.
The invention has been devised for solving the problems mentioned above and its second object is to provide an image forming apparatus wherein a large amount of toners do not stay on the blade and thereby stabilized cleaning can be performed even when the blade of the cleaning means can not be slanted fully in its operation condition and single color toner image formations are repeated for the large specified number of times under the condition of a single color mode.